访问控制（可见性）

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对属性或方法的访问控制，是通过在前面添加关键字 public（公有），protected（受保护）或 private（私有）来实现的。被定义为公有的类成员可以在任何地方被访问。被定义为受保护的类成员则可以被其自身以及其子类和父类访问。被定义为私有的类成员则只能被其定义所在的类访问。

属性的访问控制

类属性必须定义为公有，受保护，私有之一。如果用 var 定义，则被视为公有。

Example #1 属性声明

  <?php
 
 class  MyClass
 {
    public  $public  =  'Public' ;
    protected  $protected  =  'Protected' ;
    private  $private  =  'Private' ;

    function  printHello ()
    {
        echo  $this -> public ;
        echo  $this -> protected ;
        echo  $this -> private ;
    }
}

 $obj  = new  MyClass ();
echo  $obj -> public ;  // 这行能被正常执行
 echo  $obj -> protected ;  // 这行会产生一个致命错误
 echo  $obj -> private ;  // 这行也会产生一个致命错误
 $obj -> printHello ();  // 输出 Public、Protected 和 Private



 class  MyClass2  extends  MyClass
 {
     // 可以对 public 和 protected 进行重定义，但 private 而不能
     protected  $protected  =  'Protected2' ;

    function  printHello ()
    {
        echo  $this -> public ;
        echo  $this -> protected ;
        echo  $this -> private ;
    }
}

 $obj2  = new  MyClass2 ();
echo  $obj2 -> public ;  // 这行能被正常执行
 echo  $obj2 -> private ;  // 未定义 private
 echo  $obj2 -> protected ;  // 这行会产生一个致命错误
 $obj2 -> printHello ();  // 输出 Public、Protected2 和 Undefined

 ?>

Note: 为了兼容性考虑，在 PHP 4 中使用 var 关键字对变量进行定义的方法在 PHP 5 中仍然有效（只是作为 public 关键字的一个别名）。在 PHP 5.1.3 之前的版本，该语法会产生一个 E_STRICT 警告。

方法的访问控制

类中的方法可以被定义为公有，私有或受保护。如果没有设置这些关键字，则该方法默认为公有。

Example #2 方法声明

  <?php
 
 class  MyClass
 {
     // 声明一个公有的构造函数
     public function  __construct () { }

     // 声明一个公有的方法
     public function  MyPublic () { }

     // 声明一个受保护的方法
     protected function  MyProtected () { }

     // 声明一个私有的方法
     private function  MyPrivate () { }

     // 此方法为公有
     function  Foo ()
    {
         $this -> MyPublic ();
         $this -> MyProtected ();
         $this -> MyPrivate ();
    }
}

 $myclass  = new  MyClass ;
 $myclass -> MyPublic ();  // 这行能被正常执行
 $myclass -> MyProtected ();  // 这行会产生一个致命错误
 $myclass -> MyPrivate ();  // 这行会产生一个致命错误
 $myclass -> Foo ();  // 公有，受保护，私有都可以执行



 class  MyClass2  extends  MyClass
 {
     // 此方法为公有
     function  Foo2 ()
    {
         $this -> MyPublic ();
         $this -> MyProtected ();
         $this -> MyPrivate ();  // 这行会产生一个致命错误
     }
}

 $myclass2  = new  MyClass2 ;
 $myclass2 -> MyPublic ();  // 这行能被正常执行
 $myclass2 -> Foo2 ();  // 公有的和受保护的都可执行，但私有的不行

 class  Bar 
 {
    public function  test () {
         $this -> testPrivate ();
         $this -> testPublic ();
    }

    public function  testPublic () {
        echo  "Bar::testPublic\n" ;
    }
    
    private function  testPrivate () {
        echo  "Bar::testPrivate\n" ;
    }
}

class  Foo  extends  Bar 
 {
    public function  testPublic () {
        echo  "Foo::testPublic\n" ;
    }
    
    private function  testPrivate () {
        echo  "Foo::testPrivate\n" ;
    }
}

 $myFoo  = new  foo ();
 $myFoo -> test ();  // Bar::testPrivate 
                // Foo::testPublic
 ?>

其它对象的访问控制

同一个类的对象即使不是同一个实例也可以互相访问对方的私有与受保护成员。这是由于在这些对象的内部具体实现的细节都是已知的。

Example #3 访问同一个对象类型的私有成员

  <?php
 class  Test
 {
    private  $foo ;

    public function  __construct ( $foo )
    {
         $this -> foo  =  $foo ;
    }

    private function  bar ()
    {
        echo  'Accessed the private method.' ;
    }

    public function  baz ( Test $other )
    {
         // We can change the private property:
         $other -> foo  =  'hello' ;
         var_dump ( $other -> foo );

         // We can also call the private method:
         $other -> bar ();
    }
}

 $test  = new  Test ( 'test' );

 $test -> baz (new  Test ( 'other' ));
 ?>

以上例程会输出：

string(5) "hello"
Accessed the private method.

用户评论:

[#1] Anonymous [2015-09-28 12:39:09]

A useful extension to Example #2:

<?php class Base { public function test() { $this->testPublic(); self::testPublic(); $this->testProtected(); self::testProtected(); $this->testPrivate(); self::testPrivate(); } public function testPublic() { echo "Base::testPublic\n"; } protected function testProtected() { echo "Base::testProtected\n"; } private function testPrivate() { echo "Base::testPrivate\n"; } } class Ext extends Base { public function testPublic() { echo "Ext::testPublic\n"; } protected function testProtected() { echo "Ext::testProtected\n"; } private function testPrivate() { echo "Ext::testPrivate\n"; } } $ext = new Ext(); $ext->test(); // gives: // Ext::testPublic // Ext::testProtected // Base::testPrivate ?>

[#2] Anonymous [2015-09-28 12:25:24]

I believe
<?php $myFoo = new foo();?>
in example 2 should be
<?php $myFoo = new Foo();?>.

[#3] willbrownsberger at gmail dot com [2015-09-03 01:20:54]

Just wanted to share a trap for the unwary.  Where there are several layers of object assignments, setting the bottom object's properties as private will prevent its exposure.  However, if the bottom object has public properties, intermediate objects which are themselves set as private but are derived from the bottom object can inadvertently be exposed to updates.

This follows logically from the reference model in php ( http://php.net/manual/en/language.oop5.references.php ), but can yield a result that is surprising until one gets the reference model.  The following example demonstrates the phenomenon.

// underlying class for offering database results to other objects
// __construct method yields public results -- bottom object in example

class database_result {
public $column1;
public function __construct() {
// . . . database access . . .
$this->column1 = 'foo';
}
}

// application dictionary accesses database and caches results
// for application objects -- this is the second layer  in the example

class  dictionary {
private $reference_object;
public function __construct (){
$this->reference_object = new database_result;
}

public function get_reference_object() {
return ( $this->reference_object );
}
}

$dictionary = new dictionary;

$pointer_to_dictionary = $dictionary;

//  now set up a client class that will use a working copy of the dictionary -- this is the third layer in the example

class dictionary_user {
private $pointer_to_dictionary;
public function __construct () {
global $dictionary;
// $this->pointer_to_dictionary = $dictionary->reference_object;
// Fatal error: Cannot access private property dictionary::$reference_object in /var/www/html/index.php  . . .
// still cannot directly access dictionary properties even in this context, except through getter
$this->pointer_to_dictionary = $dictionary->get_reference_object();
}

// however, can now operate on dictionary through the pointer
public function set_pointer_to_dictionary ( $value ) {
$this->pointer_to_dictionary->column1 = $value;
}

public function get_pointer_to_reference_object(){
return ($this->pointer_to_dictionary);
}
}

$dictionary_user = new dictionary_user;
$dictionary_user->set_pointer_to_dictionary ( 'foochanged' );
echo ('<br/>');
var_dump ( $dictionary_user->get_pointer_to_reference_object()); echo '<br/>';
// object(database_result)#2 (1) { ["column1"]=> string(10) "foochanged" } -- of course, the user object is changed
var_dump ( $dictionary->get_reference_object() );
// object(database_result)#2 (1) { ["column1"]=> string(10) "foochanged" }  -- however, the private dictionary object is also now corrupted!
// Note:  If the underlying database result object $column1 as private, this will cause set_pointer_to_dictionary to generate the usual fatal error
// but  making the bottom object private may defeat its purpose of exposing results.

[#4] pgl at yoyo dot org [2015-06-30 11:09:18]

Just a quick note that it's possible to declare visibility for multiple properties at the same time, by separating them by commas.

eg:

<?php class a { protected $a, $b; public $c, $d; private $e, $f; } ?>

[#5] hasan4it at gmail dot com [2014-03-04 07:19:00]

class MyClass
{
    public $public = 'Public';
    protected $protected = 'Protected';
    private $private = 'Private';

    function printHello()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj = new MyClass();

print_r($obj)

it gives me the value of all variables
How can I hide this?

[#6] thcdesigning at gmail dot com [2013-11-26 12:50:58]

Private or not private?
I get baffled whenever I see this kind of an example.

<?php

class vessel{
private $things = array();

public function setThing($things){
$this->things = $things;
}

public function getThing($obj){
return $obj->things;
}
}

class smallVessel{
private $things = array();

public function setThing($things){
$this->things = $things;
}

public function getThing($obj){
return $obj->things;
}
}

$basket = new vessel();
$bucket = new vessel();
$bowl = new smallVessel();

$basket->setThing(array('wine' , 'water' , 'sugar'));

// returns the contents inside basket unexpectedly
print_r($bucket->getThing($basket));

// returns error, quite rightly so!
print_r($bowl ->getThing($basket));

[#7] aluciffer at hotmail dot com [2013-07-21 14:08:13]

As far as it regards the properties of objects, visibility is, yes, as the examples show.
Private, protected methods are not accessible via syntax $a->protectedVar, however their values are still (php 5.3.26) accessible through a number of other methods (serializing, converting to array, and nevertheless using the ReflectionClass methods).
As it was pointed out and such as in the example below:
<?php echo "PHP Version: ".phpversion()."\n"; class Foo {    private   $bar  = "value of private var";    protected $bar2 = "value of protected var";    public    $bar3 = "value of public var"; } $obj = new Foo; echo serialize($obj) . "\n"; print_r($obj); print_r((array)$obj); echo ($obj->bar3) . "\n"; echo ($obj->bar2) . "\n"; echo ($obj->bar) . "\n"; ?>

It will output:

PHP Version: 5.3.26
O:3:"Foo":3:{s:8:"Foobar";s:20:"value of private var ";s:7:"*bar2";s:22:"value of protected var";s:4:"bar3";s:19:"value of public var";}
Foo Object
(
    [bar:Foo:private] => value of private var
    [bar2:protected] => value of protected var
    [bar3] => value of public var
)

Array
(
    [Foobar] => value of private var
    [*bar2] => value of protected var
    [bar3] => value of public var
)
value of public var

While the last two lines, accessing directly the private and protected object properties (bar2 and bar), will throw out fatal errors like:
PHP Fatal error:  Cannot access private property Foo::$bar
and
PHP Fatal error:  Cannot access protected property Foo::$bar2

[#8] andrei at leapingbytes dot net [2012-12-20 09:50:54]

Interestingly enough, PHP does very reasonable job in regards to interaction between classes and plain functions (even ones defined in the same file as the class)

<?php class Test { private function foo() { echo "Foo" . PHP_EOL; } protected function bar() { echo "bar" . PHP_EOL; } static function foobar($test) { $test->bar(); // works $test->foo(); // works } } function simple_function() { $test = new Test(); $test->bar(); // does not work $test->foo(); // does not work Test::foobar($test); // works } simple_function(); ?>

[#9] alexaulbach at mayflower dot de [2012-11-06 13:32:34]

<?php error_reporting(E_ALL | E_STRICT | E_ERROR | E_WARNING | E_PARSE | E_COMPILE_ERROR); class A { private $private = 1; public $public = 1; function get() { return "A: $this->private , $this->public\n"; } } class B extends A { function __construct() { $this->private = 2; $this->public = 2; } function set() { $this->private = 3; $this->public = 3; } function get() { return parent::get() . "B: $this->private , $this->public\n"; } } $B = new B; echo $B->get(); echo $B->set(); echo $B->get(); ?>

?>

Result is
A: 1 , 2
B: 2 , 2
A: 1 , 3
B: 3 , 3

This is correct code and does not warn you to use any private.

"$this->private" is only in A private. If you write it in class B it's a runtime declaration of the public variable "$this->private", and PHP doesn't even warn you that you create a variable in a class without declaration, because this is normal behavior.

[#10] omega at 2093 dot es [2012-07-06 12:04:51]

This has already been noted here, but there was no clear example. Methods defined in a parent class can NOT access private methods defined in a class which inherits from them. They can access protected, though.

Example:

<?php class ParentClass { public function execute($method) { $this->$method(); } } class ChildClass extends ParentClass { private function privateMethod() { echo "hi, i'm private"; } protected function protectedMethod() { echo "hi, i'm protected"; } } $object = new ChildClass(); $object->execute('protectedMethod'); $object->execute('privateMethod'); ?>

Output:

hi, i'm protected
Fatal error: Call to private method ChildClass::privateMethod() from context 'ParentClass' in index.php on line 6

In an early approach this may seem unwanted behaviour but it actually makes sense. Private can only be accessed by the class which defines, neither parent nor children classes.

[#11] jc dot flash at gmail dot com [2012-06-21 02:31:23]

if not overwritten, self::$foo in a subclass actually refers to parent's self::$foo
<?php class one { protected static $foo = "bar"; public function change_foo($value) { self::$foo = $value; } } class two extends one { public function tell_me() { echo self::$foo; } } $first = new one; $second = new two; $second->tell_me(); // bar $first->change_foo("restaurant"); $second->tell_me(); // restaurant ?>

[#12] IgelHaut [2012-05-30 13:48:51]

<?php class test { public $public = 'Public var'; protected $protected = 'protected var'; private $private = 'Private var'; public static $static_public = 'Public static var'; protected static $static_protected = 'protected static var'; private static $static_private = 'Private static var'; } $class = new test; print_r($class); ?>

The code prints
test Object ( [public] => Public var [protected:protected] => protected var [private:test:private] => Private var )

Functions like print_r(), var_dump() and var_export() prints public, protected and private variables, but not the static variables.

[#13] wbcarts at juno dot com [2012-05-19 20:27:17]

INSIDE CODE and OUTSIDE CODE

<?php class Item { public $label; public $price; } $item = new Item(); $item->label = 'Ink-Jet Tatoo Gun'; $item->price = 49.99; ?>

Ok, that's simple enough... I got it inside and out. The big problem with this is that the Item class is COMPLETELY IGNORANT in the following ways:
* It REQUIRES OUTSIDE CODE to do all the work AND to know what and how to do it -- huge mistake.
* OUTSIDE CODE can cast Item properties to any other PHP types (booleans, integers, floats, strings, arrays, and objects etc.) -- another huge mistake.

Note: we did it correctly above, but what if someone made an array for $price? FYI: PHP has no clue what we mean by an Item, especially by the terms of our class definition above. To PHP, our Item is something with two properties (mutable in every way) and that's it. As far as PHP is concerned, we can pack the entire set of Britannica Encyclopedias into the price slot. When that happens, we no longer have what we expect an Item to be.

INSIDE CODE should keep the integrity of the object. For example, our class definition should keep $label a string and $price a float -- which means only strings can come IN and OUT of the class for label, and only floats can come IN and OUT of the class for price.

<?php class Item { protected $label = 'Unknown Item'; // Rule 1 - protected. protected $price = 0.0; // Rule 1 - protected. public function getLabel() { // Rule 2 - public function. return $this->label; // Rule 3 - string OUT for $label. } public function getPrice() { // Rule 2 - public function. return $this->price; // Rule 4 - float OUT for $price. } public function setLabel($label) // Rule 2 - public function. { if(is_string($label)) { $this->label = (string)$label; // Rule 3 - string IN for $label. } } public function setPrice($price) // Rule 2 - public function. { if(is_numeric($price)) { $this->price = (float)$price; // Rule 4 - float IN for $price. } } } ?>

Now there is nothing OUTSIDE CODE can do to obscure the INSIDES of an Item. In other words, every instance of Item will always look and behave like any other Item complete with a label and a price, AND you can group them together and they will interact without disruption. Even though there is room for improvement, the basics are there, and PHP will not hassle you... which means you can keep your hair!

[#14] briank at kappacs dot com [2011-11-10 12:20:08]

To make (some) object members read-only outside of the class (revisited using PHP 5 magic __get):

<?php class ReadOnlyMembers { private $reallyPrivate; private $justReadOnly; function __construct () { $this->reallyPrivate = 'secret'; $this->justReadOnly = 'read only'; } function __get ($what) { switch ($what) { case 'justReadOnly': return $this->$what; default: # Generate an error, throw an exception, or ... return null; } } function __isset ($what) { $val = $this->__get($what); return isset($val); } } $rom = new ReadOnlyMembers(); var_dump($rom->justReadOnly); // string(9) "read only" $rom->justReadOnly = 'new value'; // Fatal error var_dump($rom->reallyPrivate); // Fatal error ?>

[#15] php at stage-slash-solutions dot com [2011-07-10 15:21:59]

access a protected property:

<?php //Some library I am not allowed to change: abstract class a { protected $foo; } class aa extends a { function setFoo($afoo) { $this->foo = $afoo; } } ?>

if you get an instance of aa and need access to $foo:

<?php class b extends a { function getFoo($ainstance) { return $ainstance->foo; } } $aainstance=someexternalfunction(); $binstance=new b; $aafoo=$binstance->getFoo($aainstance); ?>

[#16] Marce! [2009-09-25 03:04:23]

Please note that protected methods are also available from sibling classes as long as the method is declared in the common parent. This may also be an abstract method.

In the below example Bar knows about the existence of _test() in Foo because they inherited this method from the same parent. It does not matter that it was abstract in the parent.

<?php abstract class Base { abstract protected function _test(); } class Bar extends Base { protected function _test() { } public function TestFoo() { $c = new Foo(); $c->_test(); } } class Foo extends Base { protected function _test() { echo 'Foo'; } } $bar = new Bar(); $bar->TestFoo(); // result: Foo ?>

[#17] imran at phptrack dot com [2009-08-18 14:06:14]

Some Method Overriding rules :

1. In the overriding, the method names and arguments (arg??s) must be same.

Example:
class p { public function getName(){} }
class c extends P{ public function getName(){} }

2. final methods can??t be overridden.

3. private methods never participate in the in the overriding because these methods are not visible in the child classes.

Example:
class a {
private  function my(){
print "parent:my";
}
public function getmy(){
$this->my();
}
}
class b extends a{
private  function my(){
print "base:my";
}
}
$x = new b();
$x->getmy(); // parent:my

4. While overriding decreasing access specifier is not allowed

class a {
public  function my(){
print "parent:my";
}

}
class b extends a{
private  function my(){
print "base:my";
}
}
//Fatal error:  Access level to b::my() must be public (as in class a)

[#18] a dot schaffhirt at sedna-soft dot de [2009-06-29 00:27:58]

If you miss the "package" keyword in PHP in order to allow access between certain classes without their members being public, you can utilize the fact, that in PHP the protected keyword allows access to both subclasses and superclasses.

So you can use this simple pattern:

<?php abstract class Dispatcher { protected function &accessProperty (self $pObj, $pName) { return $pObj->$pName; } protected function invokeMethod ($pObj, $pName, $pArgs) { return call_user_func_array(array($pObj, $pName), $pArgs); } } ?>

The classes that should be privileged to each other simply extend this dispatcher:

<?php class Person extends Dispatcher { private $name; protected $phoneNumbers; public function __construct ($pName) { $this->name = $pName; $this->phoneNumbers = array(); } public function addNumber (PhoneNumber $pNumber, $pLabel) { $this->phoneNumbers[$pLabel] = $pNumber; // this does not work, because "owner" is protected: // $pNumber->owner = $this; // instead, we get a reference from the dispatcher: $p =& $this->accessProperty($pNumber, "owner"); // ... and change that: $p = $this; } public function call ($pLabel) { // this does not work since "call" is protected: // $this->phoneNumbers[$pLabel]->call(); // instead, we dispatch the call request: $this->invokeMethod($this->phoneNumbers[$pLabel], "call", array()); } } class PhoneNumber extends Dispatcher { private $countryCode; private $areaCode; private $number; protected $owner; public function __construct ($pCountryCode, $pAreaCode, $pNumber) { $this->countryCode = $pCountryCode; $this->areaCode = $pAreaCode; $this->number = $pNumber; } protected function call () { echo("calling " . $this->countryCode . "-" . $this->areaCode . "-" . $this->number . "\n"); } } $person = new Person("John Doe"); $number1 = new PhoneNumber(12, 345, 67890); $number2 = new PhoneNumber(34, 5678, 90123); $person->addNumber($number1, "home"); $person->addNumber($number2, "office"); $person->call("home"); ?>

Without this pattern you would have to make $owner and call() public in PhoneNumber.

Best regards,

[#19] what at ever dot com [2008-12-04 05:15:17]

If you have problems with overriding private methods in extended classes, read this:)

The manual says that "Private limits visibility only to the class that defines the item". That means extended children classes do not see the private methods of parent class and vice versa also.

As a result, parents and children can have different implementations of the "same" private methods, depending on where you call them (e.g. parent or child class instance). Why? Because private methods are visible only for the class that defines them and the child class does not see the parent's private methods. If the child doesn't see the parent's private methods, the child can't override them. Scopes are different. In other words -- each class has a private set of private variables that no-one else has access to.

A sample demonstrating the percularities of private methods when extending classes:

<?php abstract class base { public function inherited() { $this->overridden(); } private function overridden() { echo 'base'; } } class child extends base { private function overridden() { echo 'child'; } } $test = new child(); $test->inherited(); ?>

Output will be "base".

If you want the inherited methods to use overridden functionality in extended classes but public sounds too loose, use protected. That's what it is for:)

A sample that works as intended:

<?php abstract class base { public function inherited() { $this->overridden(); } protected function overridden() { echo 'base'; } } class child extends base { protected function overridden() { echo 'child'; } } $test = new child(); $test->inherited(); ?>
Output will be "child".

[#20] Joshua Watt [2007-05-29 12:09:08]

I couldn't find this documented anywhere, but you can access protected and private member varaibles in different instance of the same class, just as you would expect

i.e.

<?php class A { protected $prot; private $priv; public function __construct($a, $b) { $this->prot = $a; $this->priv = $b; } public function print_other(A $other) { echo $other->prot; echo $other->priv; } } class B extends A { } $a = new A("a_protected", "a_private"); $other_a = new A("other_a_protected", "other_a_private"); $b = new B("b_protected", "ba_private"); $other_a->print_other($a); //echoes a_protected and a_private $other_a->print_other($b); //echoes b_protected and ba_private $b->print_other($a); //echoes a_protected and a_private ?>

[#21] stephane at harobed dot org [2006-08-23 02:22:49]

A class A static public function can access to class A private function :

<?php class A { private function foo() { print("bar"); } static public function bar($a) { $a->foo(); } } $a = new A(); A::bar($a); ?>

It's working.

[#22] r dot wilczek at web-appz dot de [2006-01-05 05:11:33]

Beware: Visibility works on a per-class-base and does not prevent instances of the same class accessing each others properties!

<?php class Foo { private $bar; public function debugBar(Foo $object) { // this does NOT violate visibility although $bar is private echo $object->bar, "\n"; } public function setBar($value) { // Neccessary method, for $bar is invisible outside the class $this->bar = $value; } public function setForeignBar(Foo $object, $value) { // this does NOT violate visibility! $object->bar = $value; } } $a = new Foo(); $b = new Foo(); $a->setBar(1); $b->setBar(2); $a->debugBar($b); // 2 $b->debugBar($a); // 1 $a->setForeignBar($b, 3); $b->setForeignBar($a, 4); $a->debugBar($b); // 3 $b->debugBar($a); // 4 ?>